Review trackpad scroll_direction() handling.

[djowel]

This is related to the so-called trackpad "Natural scrolling".

In WIndows, scroll_direction() is obtained from the registry only once. What if the user changes the control panel settings while the app is running?

In Linux,it is not implemented and simly returns {+1.0f, +1.0f}

See if this can be done better.

[djowel]

Partial implementation for Linux in b7b79c34676cb89b800fdc8960cb311e205e1db8

Same timed call should work for Windows as well.